Modeling Laser Generated Shock Damage and Thermo-Mechanical Chaos in Nanoparticles
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Modeling Laser Generated Shock Damage and Thermo-Mechanical Chaos in Nanoparticles. Bernard S. Gerstman Department of Physics Florida International University Miami, FL 33199, 305-348-3115 [email protected]
ABSTRACT Continual advances in laser technology lead to shorter pulses and higher energies. As the duration of a laser pulse shortens, different physical mechanisms become important in determining the thermo-mechanical response of an absorbing particle. These thermomechanical responses fall into the general category of thermal heating (temperature rise), vaporization, and shock wave formation. Our theoretical work has produced a computational model that allows the quantitative calculation of all of these responses for a laser of any pulse duration or energy, absorbed by a particle of any size. We find that for relatively long pulses, particle damage occurs most easily, i.e. at the least pulse energy, due to thermal effects. As the pulse duration shortens, explosive vaporization can dominate as the primary damage mechanism. For short pulses, shock wave production becomes the dominant damage mechanism. We describe how the relative terms of “short” and “long” pulse duration can be determined from knowledge of the thermo-mechanical properties of the absorber. Conversely, when the thermo-mechanical properties are not known, we explain how our theoretical work leads suggests an experimental technique that allows measurement of these absorber properties. This technique is applicable to extremely small particles that present difficulties for thermo-mechanical measurements. Finally, we show computational evidence of chaotic behavioral response of the absorber. This results in some laser pulse durations and energies that cause anomalously small shock waves, whereas other durations and energies cause surprisingly large and damaging responses. INTRODUCTION Bubble formation and shock wave generation by photoacoustic means have been the focus of recent works by a variety of authors.1-7 A main reason for this interest is that pulsed lasers are used on an increasing basis in medical and commercial applications. A true quantitative understanding of the various physical processes involved in the interaction of laser light with micro and nanoparticles is necessary to maximize progress in material science research and development. This deep level of understanding is also crucial in the biomedical field with many direct implications, such as setting safety standards8 and for maximizing the benefit and minimizing the risk in intraocular laser surgery.9,10 Applications in applied physics and material science are directly related to investigations of stress damage in polymers11 and determining the thermomechanical properties of micro and nanoparticles used to anchor long molecules such as DNA for manipulation. From the standpoint of fundamental physics, the subject is a highly non-
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linear problem with close connections to systems found in acoustics, hydrodynamics, and bubble dynamics.12 The small size of micron and n
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